Color images are commonly obtained in the silver halide photographic art by reaction between the development product of a silver halide developing agent (e.g., oxidized aromatic primary amine developing agent) and a color forming compound commonly referred to as a coupler. The reaction between the coupler and oxidized developing agent results in coupling of the oxidized developing agent to the coupler at a reactive site on the coupler, known as the coupling position, and yields a dye. The subtractive process of color formation is ordinarily employed in color photographic elements, and the dyes produced by coupling are usually cyan, magenta, or yellow dyes which are formed in or adjacent to silver halide emulsion layers sensitive to red, green, or blue radiation, respectively.
Couplers well known for forming magenta dyes include, e.g., pyrazolones and bicyclic pyrazoloazoles, as described, for example, in U.S. Pat. Nos. 3,725,067; 3,810,761; 4,443,536; 4,540,654; and 4,621,046.
Well known couplers usually employed for forming cyan dyes include, e.g., substituted phenols and naphthols, as described, for example, in U.S. Pat. Nos. 2,367,531; 2,423,730; 2,474,293; 2,772,162; 2,895,826; 3,002,836; 3,034,892; 3,041,236; 3,998,642; 4,333,999; and 4,443,536.
Many couplers well known for forming yellow dyes contain an open-chain ketomethylene group in the coupling moleties. One class of such couplers comprises acylacetanilides, such as pivalylacetanilides and benzoylacetanilides, described, for example, in U.S. Pat. Nos. 2,407,210; 3,265,506; 3,408,194; 3,894,875; and 4,157,919.
However, such known couplers often have drawbacks.
One common drawback is the relatively high equivalent weight of many couplers. The term, "equivalent weight", as used herein is equal to the molecular weight of the coupler divided by the number of efficiently reactive coupling moieties in the coupler molecule. Each efficiently reactive coupling moiety is capable of reacting with oxidized developing agent to form a colored dye moiety. The higher the equivalent weight of the coupler is, the larger is the mass of coupler that must be included in a photographic element layer in order to be able to produce the desired amount of developed image dye optical density. The need for a larger mass of coupler in a layer results in a thicker layer, which inherently reduces the transparency and the optical sharpness of the layer. Thus, lower equivalent weight couplers allow for thinner, more transparent, optically sharper layers. Unfortunately, the overall mass of a coupler molecule must be relatively large in order to provide sufficient organic ballast to properly suspend the coupler molecules in droplets of high boiling organic liquid, referred to as coupler solvent, which are dispersed in the desired layer of the photographic element, and thereby anchor the coupler in the layer and prevent it from diffusing to adjacent layers or out of the element during processing with various aqueous processing liquids. Thus, the needs for lower equivalent weight and sufficient organic ballast are at apparent cross-purposes.
There is therefore a continuing need for new dye-forming couplers that can minimize the drawbacks described above, i.e., that have relatively low equivalent weight, while at the same time having relatively high molecular weight to provide sufficient organic ballast for proper incorporation and anchoring in photographic element layers. Of course, the couplers should also exhibit all the other characteristics desirable for good photographic performance.